During a critical period in early life, responses of neurons in the visual cortex exhibit a reliable and very rapid plasticity, apparent in a day or less and almost complete in 2 days. Structural changes found to date in the inputs to the visual cortex take 2-3 times longer for similar full expression. I will determine whether changes in the postsynaptic components of synapses made by the geniculocortical afferents take place with a time course that is as rapid as the physiological changes. These experiments have the potential to reveal the early stages of the mechanisms by which neural activity causes connections in the developing mammalian nervous system to reorganize. These mechanisms are thought to be responsible for allowing the central visual system to develop with its normal great precision. In addition, defining the alterations in cortical function and structure caused by sensory deprivation may help explain some of the pathological processes at work in conditions such as amblyopia, congenital cortical blindness and cerebral palsy.